A tooth damaged by decaying, etc. can be restored by using a dental restorative called composite resin that contains a polymerizable monomer such as a methacrylate compound and a filler as chief components. In recent years, however, a photocurable composite resin blended with a photopolymerization initiator has been widely used since it can be used requiring simple operation. The composite resin is, usually, filled in the cavity of a tooth and is, thereafter, cured by polymerization. However, the composite resin by itself has no adhering property to the teeth and, therefore, a dental adhesive is used in combination with the composite resin. The adhesive must exhibit an adhesive strength large enough to overcome the internal stress that generates accompanying the curing, i.e., to overcome the tensile stress that occurs on the interface between the teeth and the composite resin. Otherwise, the composite resin may peel off after used for extended periods of time under severe oral environment and, besides, a gap may develop in the interface between the teeth and the composite resin permitting germs to infiltrate to adversely affect the dental pulp.
A bracket for correcting irregular teeth, too, is adhered to the teeth by using the dental adhesive. The adhesive for the bracket, too, has been replaced by the photocurable adhesive owing to its simplicity of use. The photocurable adhesive for the bracket has an advantage of excellent operability in that the adhesive can be photocured and adhered at any timing after the bracket has been positioned on the tooth surface. A generally employed method of adhesion comprises applying the adhesive for the bracket onto the predetermined surface of the bracket, and closely adhering the applied surface onto the tooth surface followed by photocuring.
The dental adhesive used for the composite resin or the bracket contains chiefly an acidic group-containing polymerizable monomer, a polymerizable monomer and a polymerization initiator as constituent components involving, however, a problem in that none of them can adhere to the teeth to a sufficient degree. In order to firmly fix the composite resin or the bracket to the dentin, therefore, the dentin has, usually, been pre-treated through two steps as described below prior to applying the adhesive for the composite resin or the adhesive for the bracket:
(1) Hard tooth is etched by applying an aqueous solution (etching agent) of an acid such as phosphoric acid, citric acid or maleic acid to the surface of the tooth; and
(2) After etching, a solution (permeation accelerator, also called primer) containing an amphipatic monomer such as hydroxyethyl methacrylate (HEMA) and an organic solvent as chief components is applied to have the permeation accelerator permeate into the teeth.
The hard tissue of a tooth consists of an enamel and a dentin. Clinically, therefore, the adhesion must be effected to both of them. In general, the adhesion to the enamel is said to be a macroscopic mechanical fitting in which the adhesive permeates and cures in a coarse surface formed by deliming with an acidic aqueous solution whereas the adhesion to the dentin is said to be a microscopic mechanical fitting in which the adhesive permeates and cures among the fine gaps of spongy collagen fiber exposed on the surface of the tooth due to deliming. Here, however, the adhesive does not permeate into the collagen fiber so easily as into the surface of the enamel. Therefore, the etching is effected to improve adhesion to the enamel and, thereafter, the permeation treatment is effected by using the permeation accelerator (primer) to improve adhesion to the dentin.
According to the prior art as described above, the pre-treatment had to be effected in two steps prior to applying the dental adhesive in order to attain favorable adhesion to both the enamel and the dentin requiring, however, a cumbersome operation.
In order to reduce the complexity of operation, there has already been known a primer composition having both the deliming function (etching function) for the enamel and the dentin, and permeation property to the dentin (see, for example, patent document 1 and patent document 2).    Patent document 1: JP-A-6-9327    Patent document 2: JP-A-6-24928
That is, the above primer composition contains, as chief components, a polymerizable monomer having an acidic group such as phosphoric acid group or carboxylic acid group exhibiting teeth-deliming property as well as affinity to the dentin, and water which is necessary for deliming the teeth. The primer composition has self-etching property as well as permeation property to the dentin, and is capable of effecting both the etching and the permeation acceleration treatment to the dentin by using an acidic aqueous solution in one step of pre-treatment. In particular, the primer composition of the patent document 2 is blended with a polyvalent metal compound that elutes out polyvalent ions, such as an iron compound, and makes it possible to obtain a particularly high adhesive strength.
The above permeation accelerator or the primer composition itself is not, usually, blended with the polymerization initiator. At the time when the adhesive for the composite resin or the adhesive for the bracket applied thereon undergoes the photocuring reaction, however, radicals formed by the adhesive cause the polymerizable monomer contained in the permeation accelerator or in the primer composition to be cured by polymerization. Therefore, the adhesive is firmly adhered to the tooth.
The applicant has proposed a dental adhesive that strongly adheres to both the enamel and the dentin through a simple operation without requiring the pre-treatment (see patent document 3).    Patent document 3: JP-A-10-236912
The above dental adhesive contains, as part of the polymerizable monomer, water necessary for teeth-deliming and the polymerization initiator in addition to the acidic group-containing polymerizable monomer used in the above primer composition, and is, further, blended with a filler (e.g., fluoroaluminosilicate glass) having polyvalent metal ion-eluting property. That is, the dental adhesive has the same functions (function for teeth-deliming and function for accelerating the permeation into the dentin) as those of the above-mentioned primer composition. At the time of curing, further, the dental adhesive induces ionic crosslinking due to the action of the acidic group-containing polymerizable monomer, water and polyvalent metal ion-eluting filler in addition to the radical polymerization of the polymerizable monomer, strongly adhering to both the enamel and the dentin due to the synergistic action of the radical polymerization and the ionic crosslinking. As a result, the composite resin can be firmly adhered thereon.